In a vehicle having an automatic transmission, a clutch assembly engages or couples a rotating shaft such as an engine crankshaft with a stationary shaft such as a driveshaft in order to transmit power from the engine to the drive wheels. Likewise, the clutch assembly disengages the respective shafts to interrupt the power transfer and permit smooth shifting between, for example, the various gears of a planetary gear set. Hydraulic clutch assemblies or hydraulic clutches in particular are hydraulically-actuated torque-transmitting devices typically having a series of friction elements arranged in a clutch pack within a clutch housing. The clutch pack is actuated by a clutch-apply piston disposed within a clutch-apply cavity portion of the housing, the piston being powered or energized by a supply of non-compressible hydraulic fluid. When hydraulic clutch pressure is reduced, the clutch is released or disengaged, and when clutch pressure is increased, the clutch is actuated or engaged.
In a rotating clutch assembly, a clutch housing having a clutch-apply cavity rotates along with one of the two rotating members or shafts, while a clutch return spring helps bias the clutch-apply piston. When the clutch assembly is not engaged, clutch-apply response time may be improved by maintaining a relatively low fluid pressure within the apply cavity. However, when the clutch reaches a relatively high rotational speed, the centrifugal force imparted by the rotation of the clutch housing may induce a substantial pressure head or centrifugal apply force within the clutch-apply cavity. If the centrifugal apply force exceeds the return spring force provided by the biasing spring, inadvertent or premature application of the clutch may result.
The centrifugal apply force may be compensated for by positioning a separate balance cavity opposite the clutch-apply cavity within the clutch housing. The rotation of the clutch assembly fills the balance cavity with fluid, which can then partially drain from the balance cavity when the rotation ceases. When the balance cavity is full of fluid and the clutch assembly is rotating, the rotating fluid within the balance cavity will act to produce a centrifugal balance force or pressure that opposes or counteracts the centrifugal apply force, helping to at least partially offset or negate the apply force created by the pressure head within the clutch-apply cavity. However, when the clutch-apply cavity is attached to a rotating transmission component capable of relatively rapid acceleration and high rotational speed, such as a driveshaft connected to an electric motor within a hybrid vehicle transmission, relatively rapid fluid fill of the balance cavity is essential in order to provide a balancing opposing pressure to the rapidly increasing apply cavity pressure, and avoid premature application and therefore wear or burnout of the hydraulic clutch.